According to a direct electrostatic printing method, such as that disclosed in U.S. Pat. No. 5,036,341, a background electric field is produced between a developer sleeve and a back electrode to enable the transport of charged toner particles therebetween. A printhead structure, such as an electrode matrix provided with a plurality of selectable apertures, is interposed in the background electric field and connected to a control unit which converts an image information into a pattern of electrostatic control fields which selectively open or close the apertures, thereby permitting or restricting the transport of toner particles from the developer sleeve. The modulated stream of toner particles allowed to pass through opened apertures impinges upon an information carrier, such as paper, conveyed between the printhead structure and the back electrode, to form a visible image.
According to such a method, each single aperture is utilized to address a specific dot position of the image in a transverse direction, i.e. perpendicular to paper motion. Thus, the transversal print addressability is limited by the density of apertures through the printhead structure. For instance, a print addressability of 300 dpi requires a printhead structure having 300 apertures per inch in a transversal direction.
A new concept of direct electrostatic printing, hereinafter referred to as dot deflection control (DDC), was introduced in U.S. patent application Ser. No. 08/621,074. According to the DDC method each single aperture is used to address several dot positions on an information carrier by controlling not only the transport of toner particles through the aperture, but also their transport trajectory toward a paper, and thereby the location of the obtained dot. The DDC method increases the print addressability without requiring a larger number of apertures in the printhead structure. This is achieved by providing the printhead structure with at least two sets of deflection electrodes connected to variable deflection voltages which, during each print cycle, sequentially modify the symmetry of the electrostatic control fields to deflect the modulated stream of toner particles in predetermined deflection directions.
For instance, a DDC method performing three deflection steps per print cycle, provides a print addressability of 600 dpi utilizing a printhead structure having 200 apertures per inch.
An improved DDC method, disclosed in U.S. patent application Ser. No. 08/759,481, provides a simultaneous dot size and dot position control. This later method utilizes the deflection electrodes to influence the convergence of the modulated stream of toner particles thus controlling the dot size. According to the method, each aperture is surrounded by two deflection electrodes connected to a respective deflection voltage D1, D2, such that the electrode field generated by the control electrodes remains substantially symmetrical as long as both deflection voltages D1, D2 have the same amplitude. The amplitudes of D1 and D2 are modulated to apply converging forces on toner to obtain smaller dots. The dot position is simultaneously controlled by modulating the amplitude difference between D1 and D2. Utilizing this improved method enables 60 .mu.m dots to be obtained utilizing 160 .mu.m apertures.
With DDC, in direct electrostatic printing methods, there appears so called dot tailing when the upper limit of the print speed is attained. This is due to insufficient deflection of the last toner particles of a dot. It can thus be considered a drawback of current direct electrostatic printing methods that there is at least to a certain extent a tradeoff between print speed and image quality. Therefore, there seems to still exist a need to improve the current direct electrostatic printing method.